# std::uninitialized_fill

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< cpp‎ | memory

Defined in header `<memory>` |  |
template< class NoThrowForwardIt, class T > void uninitialized_fill( NoThrowForwardIt first, NoThrowForwardIt last, const T& value ); | (1) | (constexpr since C++26)
template< class ExecutionPolicy, class NoThrowForwardIt, class T > void uninitialized_fill( ExecutionPolicy&& policy, NoThrowForwardIt first, NoThrowForwardIt last, const T& value ); | (2) | (since C++17)

1) Copies value to an uninitialized memory area `[`first`,` last`)` as if by

for (; first != last; ++first)
`` ::new (`voidify`(*first))
`` typename std::iterator_traits<NoThrowForwardIt>::value_type(value);

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### Parameters

first, last | - | the range of the elements to initialize
value | - | the value to construct the elements with
policy | - | the execution policy to use
Type requirements
- `NoThrowForwardIt` must meet the requirements of LegacyForwardIterator.
- No increment, assignment, comparison, or indirection through valid instances of `NoThrowForwardIt` may throw exceptions. Applying &* to a `NoThrowForwardIt` value must yield a pointer to its value type.(until C++11)

### Complexity

Linear in the distance between first and last.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_raw_memory_algorithms` | `202411L` | (C++26) | constexpr for specialized memory algorithms, (1)

### Possible implementation

    template<class NoThrowForwardIt, class T>
    constexpr void uninitialized_fill(NoThrowForwardIt first, NoThrowForwardIt last,
                                      const T& value)
    {
        using V = typename std::iterator_traits<NoThrowForwardIt>::value_type;
        NoThrowForwardIt current = first;
        try
        {
            for (; current != last; ++current)
                ::new (static_cast<void*>(std::addressof(*current))) V(value);
        }
        catch (...)
        {
            for (; first != current; ++first)
                first->~V();
            throw;
        }
    }

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <memory>
    #include <string>

    int main()
    {
        const std::size_t sz = 4;
        std::allocator<std::string> alloc;
        std::string* p = alloc.allocate(sz);

        std::uninitialized_fill(p, p + sz, "Example");

        for (std::string* i = p; i != p + sz; ++i)
        {
            std::cout << *i << '\n';
            i->~basic_string<char>();
        }

        alloc.deallocate(p, sz);
    }

Output:

    Example
    Example
    Example
    Example

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 866 | C++98 | given `T` as the value type of `NoThrowForwardIt`, if T::operator new exists, the program might be ill-formed | uses global placement new instead
LWG 2433 | C++11 | this algorithm might be hijacked by overloaded operator& | uses std::addressof
LWG 3870 | C++20 | this algorithm might create objects on a const storage | kept disallowed

### See also

uninitialized_fill_n | copies an object to an uninitialized area of memory, defined by a start and a count (function template)
ranges::uninitialized_fill (C++20) | copies an object to an uninitialized area of memory, defined by a range (algorithm function object)

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